This invention relates to an aromatic hydrocarbon-formaldehyde resin, particularly a highly reactive aromatic hydrocarbon-formaldehyde resin with a higher content of reactive groups that are very reactive with compounds having active hydrogen atoms such as phenols, aliphatic saturated or unsaturated carboxylic acids, aromatic carboxylic acids, amines, etc., but with a very low content of diarylmethane components that are inactive in the reaction with the said compounds having the active hydrogen atoms, and also to a process for preparing the resin.
It is well known that the aromatic hydrocarbon-formaldehyde resin can be obtained by reaction of aromatic hydrocarbon with formaldehyde in the presence of an acid catalyst such as sulfuric acid. Generally, the aromatic hydrocarbon-formaldehyde resin is a mixture of many molecular species of oligomers and is composed of oligomers having 1 to 8 aromatic nuclei having one or more methylene bonds (--CH.sub.2 --), dimethyleneether bonds (--CH.sub.2 --O--CH.sub.2 --), and acetal bonds (--CH.sub.2 O.sub.n CH.sub.2 --) between the aromatic nuclei and with or without a methylol group (--CH.sub.2 OH), a methoxymethyl group (--CH.sub.2 OCH.sub.3), or the like at the molecular terminal. It is also known that various modified aromatic hydrocarbon resins, for example, phenol-modified aromatic hydrocarbon resins, unsaturated carboxylic acid-modified aromatic hydrocarbon resins, anilin-modified aromatic hydrocarbon resins, etc. can be obtained by reaction of the aromatic hydrocarbon-formaldehyde resin with a compound having active hydrogen atoms such as phenols, aliphatic saturated or unsaturated carboxylic acids, aromatic carboxylic acids or amines in the presence of a catalyst, and these modified aromatic hydrocarbon resins have been utilized in various fields by virtue of their characteristics.
It is also known that oxygen-containing bonds or molecular terminal groups such as CH.sub.2 OCH.sub.2 bond, [(CH.sub.2 O).sub.n CH.sub.2 ] bond, CH.sub.2 OH group, CH.sub.2 OCH.sub.3 group, etc. in the aromatic hydrocarbon-formaldehyde resin take part in the reaction of the aromatic hydrocarbon-formaldehyde resin with the compound having the active hydrogen atoms. [Angew. Chem. 60, No. 4,88-96 (1948); Die Makromolekulore Chemie, XLIII, 189-219 (1961); British Plastics, September issue, 564-567 (1965)].
The following procedure for determining the content of the reactive groups in the aromatic hydrocarbon-formaldehyde resin is usually used, for example, in case of reaction with phenol. After reaction of the aromatic hydrocarbon-formaldehyde resin with phenol, the amount of the unreacted phenol in the reaction product is measured, and the maximum amount of phenol capable of reacting with the aromatic hydrocarbon-formaldehyde resin is calculated backward therefrom as a phenol number. However, phenol has 3 reactive points in the molecule, and thus one mole of the reactive groups in the aromatic hydrocarbon-formaldehyde resin sometimes fails to correspond to one mole of phenol, that is, the content of the reactive groups in the aromatic hydrocarbon-formaldehyde resin cannot be exactly given thereby.
The present inventors have studied a procedure for determining the content of the reactive groups in the aromatic hydrocarbon-formaldehyde resin and have found "a xylenol value" as an index for the degree of reactivity of the aromatic hydrocarbon-formaldehyde resin. It has been found that the xylenol value can appropriately show the reactivity of the aromatic hydrocarbon-formaldehyde resin. The xylenol value is a value expressed in moles of 2,6-xylenol that has reacted with 1 kg of the aromatic hydrocarbon-formaldehyde resin when the aromatic hydrocarbon-formaldehyde resin is made to react with 2,6-xylenol, and calculated according to the formula given later. The higher the xylenol value, the more reactive the resin.
On the other hand, the aromatic hydrocarbon-formaldehyde resin contains components whose constituent aromatic neclei are bonded through a methylene bond, that is, oligomer components composed of diarylmethane ##STR1## The diarylmethane members fail to react with such compounds having the active hydrogen atoms, and remain as unreacted members in the modified resin, acting as a factor to lower the physical properties of the cured resin obtained from the modified resin. Thus, it is preferable that the content of the diarylmethane members are as small as possible in the resin.
Commercially available xylene formaldehyde resins, typical of the aromatic hydrocarbon formaldehyde resin, have number average molecular weights of 450 to 600 and containing 4 to 5 aromatic nuclei on the average and the xylenol value is 9.5 to 10.5 moles/kg.
The number of --CH.sub.2 OH, --CH.sub.2 OCH.sub.3 and --CH.sub.2 OCH.sub.2 OCH.sub.3 on the molecule, determined by `H-NMR` is only 1. This means that the reactive groups such as methylol groups, methoxymethyl groups, etc. are only at one molecular terminal aromatic nucleus. The said resins contain 10 to 15% by weight of the compounds whose aromatic nuclei are bonded to one another through a methylene bond and which has no substituents at the molecular terminals, i.e. diarylmethane. Such xylene-formaldehyde resins are less reactive with the such compounds having the active hydrogen atoms, and cured resins obtainable from modified xylene resins prepared by reaction with phenols, aliphatic unsaturated carboxylic acids, aromatic carboxylic acids, amines, etc. have poor physical properties such as poor mechanical strength, heat resistance, etc.